In the wafer fabrication process, after the initial preparatory steps of cleaning and dehydration baking, the wafers undergo a coating step. Wafer coating usually involves the application of two chemicals to the wafer including an adhesion-promoter such as hexamethyldisilizane (HMDS) and a photoresist chemical. After the HMDS is applied to the wafer, the wafer is spun (typically within a track system) to produce a uniform coating. After the wafer is dry, the photoresist chemical is applied to the wafer where it is then spun again to produce a uniform coating.
The coating chemicals, due to their volatile nature, are contained in closed containers. It is known in the art to provide a simple fluid delivery system which typically consists of a pumping unit and associated inlet and outlet lines. The pump draws the chemical fluid from the container or canister and delivers the chemical to the coating apparatus where it is then deposited on the wafer.
A typical chemical system, prevalent in the prior art, is schematically illustrated in FIG. 1. A feed chemical system 100 includes a fluid chemical 102, used in some processing operation (typically IC wafer fabrication), a storage tank or container 103 for holding the fluid 102, a processing system 104 for performing the processing operation and ultimately consuming the fluid, and a pumping system 106 for delivering the fluid 102 from the storage tank 103 to the processing system 104. The pumping system 106 is communicatively coupled, via a suction pipe 108, with the storage tank 103, and a discharge pipe 109 communicatively couples the pumping system 106 with the processing system 104.
In operation, the pumping system 106 draws the fluid 102 from the tank 103 via the suction pipe 108, and pumps the fluid 102 via the discharge pipe 109 to the processing system 104. The pumping system continues to draw on the tank until the level of fluid contained in the tank reaches a predetermined level. When the fluid level in the tank reaches this level, the tank is considered "empty" and the process operation must be halted, the pump shut down and the tank either refilled, or removed and replaced.
A problem with the prior art feed chemical delivery systems is that when the fluid supply canister runs dry, chemical fluid flow is interrupted, resulting in miscoating since the pumping or processing operations are not halted immediately. In the wafer coating process operation, for example, when the chemical fluid (i.e., photo-resist material) is depleted from the storage container, the coating applicator continues to operate for a time until the situation is noticed. During this time the applicator continues to feed on the wafer surface without dispensing any photo-resist material thereby leaving bare, or irregularly or insufficiently coated areas on the surface of the wafer. The unacceptable quality of the coatings deposited on the affected wafers results in scrapping of these units, thereby increasing costs and reducing production yields. In addition, when the chemical fluid is depleted, the processing system must be halted while the storage container is either replaced by a full container or replenished with more chemical fluid. Misprocessings also occur frequently when the process system is restarted after such an interruption.
Yet another problem caused by interruption of the chemical fluid flow is slowdown in the overall manufacturing throughput. As the chemical fluid flow is depleted, the processing system must be shut down and the throughput halted. Even if the processing and pumping systems are shut down at the appropriate time and the manufacturing quality is not impaired, the production line is nevertheless halted while the chemical fluid is replenished. Clearly, as chemical fluid flow interruptions become more frequent the throughput of units processed by the particular processing system is significantly reduced.
Yet another problem found in prior art chemical feed systems is the excessive consumption of chemical fluids. As the chemical fluids are depleted from the storage tanks and the tanks replaced or refilled, the opportunity for leakage or spillage is greatly increased.
Still another common problem found in most prior art chemical feed systems is that they are not contained within explosion-proof vented enclosures. The storage tanks, for example, are frequently left in the open on the facility floor. This lack of self-containment creates two other problems. First, a greater amount of facility floor space is devoted to components of the feed chemical system. Second, the potential for explosion or fire is increased. The chemical fluid is flammable and must be contained in a properly vented explosion-proof enclosure. When the tanks are left in the open, proper precautions to ensure adequate ventilation may be overloaded. Also, the storage tanks are not likely to be contained within an explosion-proof container. Furthermore, the appropriate fire prevention equipment may not be installed proximate to the storage tanks. These also lead to regulatory violations and fines, as well as increased insurance liability costs.
Accordingly, there is a definite need in the art for a continuous feed chemical system which overcomes the problems of the prior art.